Motor vehicle air conditioning units serve to air condition the passenger compartment, frequently including a refrigerant system that functions based on the cold vapor process. The refrigeration systems in mobile applications are mostly provided with a refrigerant accumulator, which may be combined with an internal heat exchanger.
The improvement according to the invention relates to the oil recirculation device of a refrigerant accumulator.
In air conditioning units using the refrigerant R744, an internal heat exchanger is often used to enhance efficiency. The internal heat exchanger functions by supercooling the high-pressure side refrigerant. The internal heat exchanger system-internally transfers heat to the low-pressure side refrigerant, which is thereby superheated.
In vehicle air conditioning units, for reasons of space, the accumulator and the internal heat exchanger are usually combined to form one component.
The combined accumulator with the internal heat exchanger integrates the functions of both single components within one component. The combined component is preferably used in mobile R744-refrigeration systems for the air conditioning of vehicles. The refrigerant accumulator with the internal heat exchanger is disposed, on the low-pressure side, between an evaporator and a compressor and on the high-pressure side, between a gas cooler and an expansion element. In a refrigeration system or a heat pump, the accumulator is positioned downstream of the evaporator, serving to collect varying refrigerant filling quantities due to varying operational conditions and having refrigerant in reserve in order to compensate for leakage losses occurring during the maintenance interval.
Compared to the single components, the combined and, hence, compact component adapts better to the limited space in the engine compartment, also enhancing cost efficiency of the total system.
In most cases, such combined refrigerant accumulators consist of two concentric containers, the inner container serving as accumulator/collector while the internal heat exchanger is positioned in the annular space.
The refrigerant enters the accumulator and is directed through a transfer opening into an annular gap between the inner container and an outer container where the internal heat exchanger is disposed. Typically, the internal heat exchanger is a tube coil heat exchanger having tubes passed by high-pressure fluid. In the space between the tubes, the low-pressure side refrigerant flows. After the low-pressure side refrigerant has left the heat exchanger, it reaches the region of a space between the containers called a flow chamber.
Because an accumulator inevitably also removes recirculating oil from the refrigerant circuit, devices must be created in the accumulator ensuring that the oil is continuously returned to the refrigerant circuit to maintain lubrication of the compressor when the refrigeration system is operated.
From prior art, different designs of refrigerant accumulators, particularly combined with internal heat exchangers, are known.
Oil return from the collector into the refrigerant circuit is established in various ways.
According to DE 102 61 886, a collector and an internal heat exchanger are one component. An inner container functions as the collector having refrigerant in reserve. In an annular gap between the inner and an outer container a tube coil heat exchanger is disposed, which is connected to the high-pressure side of the refrigerant circuit. On the low-pressure side, the refrigerant enters the collector. In the upper range of the collector, an inlet opening of a U-tube is disposed, which leads to the bottom of the collector. There, in the 180°-bend, a little hole is made, through which oil collected in a sump of the accumulator can enter the U-tube. From there, the oil is re-entrained by gaseous refrigerant flow re-entering the system. The U-tube leads upwards entering the heat exchanger.
This solution is particularly disadvantageous due to the space requirements of the U-tube which are at the expense of the collector volume.
From U.S. Pat. No. 6,463,757, a combination component designed coaxial is known, where a collector for oil return designed annular is provided with a small hole in a bottom of the collector. Through the hole the oil can drip from the collector sump into a flow of gaseous refrigerant, which entrains the oil, transporting it to a low-pressure side outlet.
The known refrigerant accumulators are disadvantageous in that in a switched off state of the refrigeration system, refrigerant oil or liquid refrigerant of the collector sump enters the flow channel of the low-pressure side refrigerant in an uncontrolled manner until the liquid level in the accumulator and in the flow channel, or annular space, respectively, have leveled out. During start-up of the refrigeration system, the liquid refrigerant outside the accumulator container must first be evaporated. This causes increased refrigerant mass volume and reduced efficiency for a while. Only after a certain operational time, the refrigerant to be stored will again become completely deposited in the accumulator.
Depending on the liquid level in the flow channel, the danger continues that liquid refrigerant would be entrained to the low-pressure side outlet, thus flowing to the compressor through the suction line. The liquid hammer involved leads, as a rule, to a destroyed or damaged container.
The solutions using a U-tube, on the one hand, to a great extent prevent larger refrigerant quantities from being evaporated quickly, and entering the compressor in liquid state. On the other hand, space requirements of the U-tube are at the expense of the storage volume of the collector. However, because it is required that the necessary storage volume of the accumulator is minimized, particularly for vehicle air conditioning units, this solution is undesirable.
Therefore, the invention is aimed at establishing a refrigerant accumulator that, particularly at a standstill of the compressor, prevents oil and liquid refrigerant from outflowing in an uncontrolled manner from the collector chamber into the flow chamber. At the same time, the useful volume of the collector is to be enlarged or the design volume of the component be made smaller. Also, safe operation of the air conditioning unit is improved by the avoidance of an inflow of large quantities of liquid refrigerant, or oil, into the compressor.